The present disclosure generally relates to printed circuit board assemblies and, more specifically, to printed circuit board assemblies configured for handling high-current loads and methods for their production.
Modern drilling and production operations often demand acquisition of significant quantities of information relating to the downhole environment. The collection of such information is commonly referred to as “logging.” When performed during a drilling operation, the logging process is often referred to more specifically as “logging while drilling (LWD)” or “measurement while drilling (MWD).” Information collected during a logging process may include, for example, the physical location and orientation of the borehole and drilling assembly, properties of the subterranean formation penetrated by a wellbore, and/or the physical and chemical conditions present in the wellbore. More specific examples of data collected during a logging process can include, for example, resistivity, conductivity, temperature, pressure, rotation, inclination, measurement of radioactive substances, magnetic measurements, acoustic measurements, seismic activity and the like.
Considerable information may also be needed to drive a drilling tool in a desired manner, particularly when conducting a directional drilling operation under the control of surface equipment. Data similar to that of a logging operation may be used for this purpose. A drilling tool for conducting a directional drilling operation will be referred to herein as a “rotary steerable drilling tool.” Rotary steerable drilling tools may also be used in conjunction with logging while drilling operations.
A variety of sensors and equipment may be employed during logging and directional drilling operations. Many of these devices employ fairly complex electrical circuitry. To dispose the electrical circuitry in a reasonably compact operating package, printed circuit boards (PCBs) are frequently used. As used herein, the term “printed circuit board (PCB)” will refer to an electronic structure having a plurality of planar conductive pathways (i.e., “conductive lines” or “pads”) disposed laterally upon a substantially non-conductive substrate. Various electronic components (i.e., transistors, resistors, capacitors, diodes, inductors, and the like) may be mechanically supported by the PCB such that they are electrically interconnected via the planar conductive pathways to produce one or more circuits that function in a desired manner. A high-current source may further supply an electrical input to the planar conductive pathways of a PCB. The term “printed circuit board assembly (PCB assembly)” will refer herein to the combination of a high-current source in electrical communication with a PCB.
The planar conductive pathways of PCBs may be disposed upon one or both sides of the non-conductive substrate, such that the PCB is single-sided or double-sided, respectively. Multiple non-conductive substrates may also be stacked upon themselves to form a multi-layered PCB. Multi-layered PCBs may be used to establish a high density of electronic circuitry per unit of operating space.
PCBs may be manufactured by laminating a metal foil or sheet, particularly copper, onto a non-conductive substrate and then etching away portions of the metal to define the planar conductive pathways on the substrate in one or more desired locations. Such processes are often referred to as being “subtractive,” since they remove metal in the course of forming the planar conductive pathways. Alternately, an ink jet printing process may be used to define the planar conductive pathways by disposing a conductive ink on the non-conductive substrate. Such processes are often referred to as being “additive,” since they add metal in the course of forming the planar conductive pathways. With both additive and subtractive processes, it is often very difficult to vary the thickness of the planar conductive pathways in a controllable manner at different locales within a PCB.
Both additive and subtractive processes may be satisfactorily used to form planar conductive pathways that are up to about 100 microns in thickness. Planar conductive pathways within this thickness range can provide acceptable PCB functionality in many instances. However, when certain high-current sources are coupled to a PCB assembly, such thin planar conductive pathways can frequently be insufficient to provide the desired ampacity. In such instances, thicker conductive pathways can be desirable in order to increase the amount of current that may be effectively carried. In the case of planar conductive pathways formed from copper, the PCB industry typically refers to copper thicknesses greater than about 100 microns as representing “thick copper” or “heavy copper.”
Unlike planar conductive pathways suitable for carrying lower amounts of current, conductive pathway thicknesses above about 100 microns can be much more difficult to achieve. Specifically, it can be difficult to controllably deposit thicker conductive pathways with uniform thicknesses over the entirety of the PCB. It can also be difficult to solder electronic components to thicker conductive pathways. Added weight from uniformly thick conductive pathways across the entirety of the PCB can also be a concern in some high-performance applications. In some instances, thicker conductive pathways may be combined with an electrically isolated but thermally conductive plate to facilitate thermal dissipation and improve ampacity, although this technology is somewhat costly and may compromise one's ability to form multi-layered PCBs.
As an alternative to using thicker planar conductive pathways within PCBs, multi-layered PCBs may be used to distribute high-current loads between multiple layers. However, this approach has limitations as well. Specifically, the multiple layers can make soldering a challenge, and failure of any one of the multiple PCB layers can result in failure of the entire PCB. Failure within the internal layers of a multi-layered PCB can be particularly problematic, since these layers are generally unable to dissipate heat as effectively as the outer layers.
Bus bars or related structures may also be connected to a PCB assembly for withdrawing high-current loads therein. However, one still needs to provide a robust pathway for carrying electrical current to the bus bar or similar structure, so bus bar approaches may likewise be limited by problematic deposition of conductive pathways of suitable thickness, as discussed above. This limitation may be circumvented in some instances by embedding the bus bar directly in the non-conductive substrate, although specialized, high-cost manufacturing processes may be needed. In addition, there can be a significant risk of mechanical failure due to delamination or decoupling of the bus bar during thermal expansion and/or vibration under extreme operating conditions.
PCB assemblies destined for use in a downhole environment may be subject to a number of challenging operating conditions including, for example, high and varying temperatures, extreme pressures, and/or vibration, any of which can result in failure of a PCB assembly and commensurate loss of downhole monitoring and control. The extreme operating conditions of downhole environments can be particularly challenging to address when providing a PCB assembly configured for accommodating a high-current load.